A field-effect transistor generally has a structure in which a source electrode and a drain electrode are formed on a semiconductor material on a substrate and a gate electrode, etc. is formed on these electrodes via an insulating layer, and is widely used not only in integrated circuits as a logic element but also in switching elements, etc. At present, inorganic semiconductor materials mostly formed of silicon are used in the field-effect transistors. Particularly, a thin-film transistor using amorphous silicon and formed on a substrate of e.g., glass is applied to displays, etc. When such an inorganic semiconductor material is used for manufacturing field-effect transistors, treatment must be performed at high temperature and in vacuum. Thus, a high investment is required for equipment and much energy is required for manufacturing, with the result that cost is extremely increased. In addition, these materials are exposed to high temperature during a field-effect transistor manufacturing process. Thus, a substrate such as a film or a plastic substrate having insufficient heat resistance cannot be used. The application field thereof is limited.
In contrast, research and development have been made on field-effect transistors using an organic semiconductor material requiring no high-temperature treatment during a field-effect transistor manufacturing process. Use of an organic material allows manufacturing in a low-temperature process and enlarges the range of materials that can be used as a substrate. As a result, it becomes possible to realize manufacturing of field-effect transistors more flexible, lighter and more irrefrangible than conventional ones. Also, it may be possible to manufacture a large-area field-effect transistor at low cost by further applying a solution containing an organic semiconductor material and employing a technique, e.g., printing such as inkjet printing, in the manufacturing step for a field-effect transistor.
However, most of the compounds conventionally used as an organic semiconductor material are insoluble or extremely less soluble in an organic solvent. Therefore, the aforementioned inexpensive techniques such as coating and inkjet printing cannot be used. A thin film is inevitably formed on a semiconductor substrate by a relatively expensive technique such as vacuum deposition. Materials (compound) suitable for practical printing were virtually not present. Even if a material soluble in an organic solvent, the semiconductor characteristics thereof are far from a practical level. Actually, there is only a material having low carrier mobility. Nevertheless, it is important to develop a semiconductor material which enables to manufacture a semiconductor by coating and printing. Several approaches have been presently made.
Patent document 1 discloses a pentacene thin-film formed by dispersing pentacene in an organic solvent and applying the resultant dispersion solution to a silicon substrate heated at 100° C. and formation of a transistor.
Patent document 2 discloses a method for manufacturing an organic transistor using a porphyrin compound by a coating method as mentioned above.
Patent document 3 discloses field-effect transistors using benzothieno[3,2-b][1]benzoselenophene (a compound represented by the formula (1) below wherein X1 and X2 are each a selenium atom; and R1 and R2 are each a hydrogen atom) and using an aryl derivative of benzothieno[3,2-b][1]benzothiophene (a compound represented by the formula (1) below wherein X1 and X2 are each a sulfur atom; and R1 and R2 are each a hydrogen atom).
Non-Patent document 1 discloses an organic field-effect transistor using, for example, a pentacene derivative having a specific substituent introduced therein and soluble in an organic solvent.
Non-Patent document 2 discloses a field-effect transistor using an aryl derivative of benzothieno[3,2-b][1]benzoselenophene (a compound represented by the formula (1) below wherein X1 and X2 are each a selenium atom; and R1 and R2 are each a hydrogen atom).
Non-Patent documents 3 and 4 disclose a synthesis method for an alkyl derivative of benzothieno[3,2-b][1]benzothiophene (a compound represented by the formula (1) below wherein X1 and X2 are each a sulfur atom; and R1 and R2 are each a hydrogen atom).
However, the case of a field-effect transistor using the alkyl derivative above is not known.    Patent Document 1: JP-A-2005-281180    Patent Document 2: JP-A-2005-322895    Patent Document 3: WO 2006/077888    Non-Patent Document 1: J. AM. CHEM. SOC. 2005, 127, 4986-4987    Non-Patent Document 2: J. Am. Chem. Soc. 2006, 128, 3044-3050    Non-Patent Document 3: Liquid Crystals (2003), 30(5), 603-610    Non-Patent Document 4: Collect. Czech. Chem. Commun, 67(5), 645-664, 2002